Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
A 6915
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February 13, 1984
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Process of Producing Aluminum Fluoride
This invention relates to a process of producing
aluminum fluoride from aluminum hydroxide and hydrogen
fluoride in a circulating fluidized bed system including a
1uidized bed reactor, a cyclone ~eparator and a recycling
line.
It 19 ~nown to produce aluminum fluoride from
fluoride-containing materials by a reaction with mlneral
acids and a subsequent crystallization (German Patent
Pu~lication 10 62 681), or by a two-stage~reactlon of
1uorine compounds ~ith alumlnum chloride (Austrian Patent
Specificatlon 130,199; German Patent S~ecification
837,690; U.S. Patent 1,831l430), or ~y a reaction of
aluminum hydroxide with a~ueou3 hydrogen fluoride (German
Patent Specifi~catlon 12 20 ~39; Laid-o~en German Appli-
cations 15~ 92;~;09~9~ 15 92 100;;~15 92 1357 and~U.S Patent
3,432,086~ or by~a~decompos;ition;of aluminum alcoholates;~
wl~h~acid~(Ge~rran Patent Publi~tAon~12~94 35B~
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Furth~rmore proc~sses using aluminum hydroxide and
hydrogen fluoride are particularly significant. The reaction
can be carried out in an aqueous phase and may be succeeded
by a crystallizing step and, if desired, by a dewatering
step (German Patent Specification 492,412)~ Alternatively~
the reaction can be carried out at elevated temperatures so
that a dry aluminum fluoride product is obtained (British
Patent Specification 328,688). Such processes are mainly
carried out in a fluidized bed, in which alumina or aluminum
hydroxide is reacted with hydrogen fluoride at elevated
temperatures (Germ~n Patent Specifications 815,343j 10 92 889;
British Patent Speciflcation 656,3747 French Patent Speci-
fications 1~011~5447 1~221~2997 1,517,952J U.S. Patent
3~057,6aO). A multi-stage process of producing aluminum
fluoride from alumlna trihydrate and partly dehydrated
alumina trihydrate by a treatment wlth a gas that contains
hydrogen fluoride is described in Lald-open German Appli-
cation 1~908~585; Laid-open French A~plication 2,002,335
and Canadian Patent Speclfication 537,403.
Those of the processes described hereinbeore
which comprise a crystallization and drying of the aluminum
fluoride are necessarily carried out ln a plurality of
stages so that they require expensive equipment, They also
involve considerable sewage~prob1ems or requlre a;circulation
o`f large guantities of solvent. In all fluidized bed
processe~, hydrogen fluoride~gas is required in a relatively
high concentratlon.
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In another process of producing aluminum fluoride,
a circulating fluidized bed i~ employed, to which liquld
hydrogen fluoride ~s directly supplied above the grate but
below the inlet for the recycled solids (German Patent
Specification 21 06 306). r~hereas that process affords the
advantage that the hydrofluoric acid is heated virtually
instantaneously through the temperature range of 60 to 250 C,
which is critical as regards corrosion, the process depends
on the availability of liquid hydrogen fluoride.
Processes of producing aluminum fluoride wlth the
ald of gases whicn contain hydrogen fluoride in a relatively
low concentration have recently become more significant.
Such gases may become available in processes involving
the pyroh~dxolysis o solid residues ln the alumlnum industry,
e.g.,o~ spentpot lining and in de~endence on the processlng
conditions with respect to concentrating treat~ent and the
subsequent evaporation contain hydrofluoride ln a concentration
of only 8 to 12 vol. % or 20 to 25: VDl./~, Said gases have a
very high water vapor content up to 70 vol.~. These condltions
give rise to the following problems in the production o~
aluminum fluoride:
An increasing reaction temperature in the aluminum
fluoride reactor and an ~llcreasing water vapor content ln
the gas will~result in an increase of the equilibrlum
hydrogen fluorlde partial pressure in the exhaust gas.
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rate (yield)
This means that the convers~on/of hydroge~ fluoride
decreases drastically as the temperature and water vapor
content of the exhaust gas increase. A decrease o the
reaction temperature in view of the still existing o~ject
to produce aluminum fluoride in the highest possible
concentration of, e.g., at least 90 wt.% AlF3 does not g~ve
the desired result because the reaction rate (kin~tics) of the
rocess is drasticall~ decreased thereby although the
rate
e~uilibrium conditions for a high conversion /of hydrogen
fluoride are improved~ The greatly decreasing reaction
rate cannot be compensated ~n practice by a longer residence
time of the solids in the reactor because sucb pxactlce
would re~uire a reactor having excesslvely large dimensions.
It is an object of the invention to provide or
the production of alu~inum fluoride a process in which gases
containin~g hydrogen fluoride in a low concentratlon can be
employed and which can be carrled out in a reactor having
i~ensions that æereasonable from a structural aspect and
rate(yield)
which permits a maximum conversion/of hydrogen fluoride in
conjunction with the recovery of a highly concentrated
product,
This object is accomplished in that the process
of the kind described first hereinbefore is carried~out
ln~accordance~wlth~;the~lnventlon ln such~a manner~that
partly~reacted aluminum hydroxide~circulated in~a cooling
aycle~ls added to~fresh~a1um1num hydroxlde~or alum1na~
hydrate in a first
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or alu~ina hydrate
stag~, in which the alum~num hydroxide/is contacted with
the hydrogen fluoride-containing exhaust gases from the
circulating fluldized bed system so as to form a gas~solids
suspension at a temperature of 150 to 250 C, whereafter
the solids are collected from the gas stream, a~ least
part of the collected solids are passed through a cooler,
cooled solids are re-contacted with fresh aluminum
or alumina hydrate
hydroxide~ and a partial stream of the collected solids
is supplied to the circulating fluidized bed ~ystem and
ts reacted therein at a temperature of at least 450 C
with hydrogen fluorlde supplied in the form of a gas in
a concentration up to 25 vol~%.
In the productlon o alumlnum ~luoride it l~ known
that alumina before its react,ion with hydrofluoric acid
having a concentration of 70 to 80 % can be partly reacted
in the exhaust gas plpe of a calcining furnace wlth the
hot exhaust gases from said furnace, which co~tain hydrogen :
fluoride, and that the exhaust gas pipe can be so dimensioned
that an intlmate contact between the exhaust gas and the
alumina is achieved (German Patent Publication 19 56 943).
But in that prior art it is not contemplated to admix
partly reacted alumLna which is c~rculated in a coollng
cycle, Such admixing 1s~not:required in the known
process bècause owing to the high concentra~ion of the
:hydrofluoric~acid which is:employed::the ratio of the'
exhaust gas~and alumina rates~is~much more~favorablè ;
a~ xegards~the he:at:balanceO: ~
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or alumina hydrat
Filter-m~ist or predried alumlnum hydroxide/may
be used as starting solids in the process in accordance
w~th the invention.
~ he proportion in which aluminum hydroxide
circulated in a cooling cycle is added will mainly depend
on the temperature of the cooled hydroxide and on the
creating
temperature of the gas / the gas-solids suspension. It is
essential to obtain a suspension at a temperature of
150 to 250 C so that the converslon of hydrogen fluoride
in the ~irst stage of the process will be con~iderably
increa~ed.
In accordance with a preferred feature of
the invention all solids collected from the gas-solids
suspension are passed through the cooler. This will ensure
that solids at an adequate rate will alway~ be available
for cooling the exhaust gas to the desired temperature.
In accordance with another preferr~d feature of
the invention the hydrogen fluoride supplied in the form
of a gas i5 reacted in the circulating fludized bed at a
temperature in the range o~ 500 to 600 C. The operation ln~
that temperature range will result in a sufficiently hlgh
the temper2ture of
reactlon rate and in an exhaust ga~/which~can be adjusted~
without a need for a cooling cycle involving an exces~ively
high coollng expenditure,
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In an advantageous embodlment of the invention,
that part of the solids whlch is to be supplied to the
circulating fluidized bed system ~s iDtroduced via the
exhaust gas stream of the circulating fluldi~ed bed
system and an interposed separator. As a result, the
exhaust gas stream is considerably cooled and an additional
reaction path is provided.
In a variant of the process in accordance with
the invention the exhaust gas stream leaving the
recycling cyclone of the circulating ~luidized bed Ls
cooled not only with alumlnum hydroxide but i8 also cooled
by a supply of cold gas~ such as alr at amblent temperature.
As a result, the rate at whlch aluminum hydroxide is
circulated in the cooling cycle can be decreased.In particular
additional influence, which is independent of the process
streams, can be exerted on the gas temperature before the
gas is contacted with the fresh starting solids.
The aluminum hydroxlde that has been collected
fro~ thei gas-solids suspension can be cooled by any
suitahle equipment. Particularly desirable equipment
consists of a fluidi~ed bed cooler, which may include a
plurality of cham~ers to be flown through by the aluminum ~
bydroxide in succession, and interconnected cooling~surfaces,
which immierse~into respective chambers and are cooled by
flowing water.
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The circulatlng fluidized bed system has the
usual desiyn and mode of operation. The fluidlzed bed
reactor may be circular or square or rectangular in cross-
section and may contain a grate or a venturi device for
the supply of tne fluidi7ing gas~ The reactor area and the
gas rate are so selected that the mean density of the
suspension in the fluidized bed reactor is in a range of
approximately 50 to 400 kg/m of reactor volume. Whereas
an orthodox fluidi~d bed is characterized by a distlnct
density step between the fluidized bed and the overlying
gas space, the entire fluidized bed reactor of a circulating
fluidized bed system is occupied by a gas-solids suspension
and the density of the ~uspension in said reactor decreases
from bottom to top. ~The mode of operation of clrculatlng
fluidized bed systems is described by L. Reh et al. in
"'Xirbelschichtprozesse f~r die Chemie- und H~tten-Industrie,
die Energieumwandlung un~ den Umweltschutz'i, Chem. Ing.
Techn.~ 55 (1983), No. 2, pages 87 to 93).
The lnvention will be explained more in detail
by way~of example wlth reference to the drawing and the
Examples.
The drawlng is a flow scheme illustrating the
proceas in accordance with the invention.
A clrculati~ng fluidlzed bed system comprising
a fluidized bed reactor 1,~a recycling cyclone 2 and a
reoycling line~3 }s~supplied through line 4 wlth~a
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g
hydrogen fluoride-containing gas, which may have been
brought to the required temperature by indirect heating or
by an admixing of combustiorl gas~ The exhaust gas f the
circulating fluidized bed system is discharged via the
passed
recycling cyclone 2 ~nd/through the venturi suspenslon
heat exchanger 5, in which solids supplied from the
fluidized be.d cooler 6 through line 7 are added to the gas.
The resulting gas-solids suspension is supplied through
line 8 to the cyclone separator 9, in which solids are
collected. Said solids are supplied through line 10 to the
circulating flutdized bed system 1, 2, 3.
In the variant inv.olving an addltional cooling
of the gas leaving the recycling cyclone 2, a cold ga9,
such as air at ambient temperature, is supplied through
line 24.
: In another venturi suspension heat exchanger 11, or alumina hydrate
fresh aluminum hydroxide/supplièd through li~e 22 and
addltional solids supplled from the fluldlzed bed cooler 6
: through line 12 and serving also to adjust the temperature
are admlxed to the exhaust gas from the cyclone separator 9
so that another gas-solids suspension is formed. That
suspension is supplied through line 13 to the cyclone
separator 14, ln which solids are collected and are supplled
through line 15 to the fluidized bed cooler 60 The exhaust
gas~is flnally purlfied ina fine~dust collector 16~(cloth
filter or~eleotrostatlc preolpltator)~and ~s then supplied
to~a~wet or dry scrubber:for a~removal of residual hydrogen :
fluoride (not.~shown).
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The fluidized bed cooler 6 contain~ two cooling
chambers 17 and 18, into wh$ch interconnected coollng
surfaces 19 extend. The fluidizing gases are supplled
through llne 20. By means of a star feeder 21, the solids
leaving the fluidized bed cooler are distributed between
lines 7 and 12.
The final product ls withdrawn from ~e circulating
fluidlzed bed syst~m through line 23.
Exam?le 1
The fluidized bed reactor 1 of the circulating
fluidized bed system is supplied through line 4 with gas
at a rate o~ 6906 m.N /h and at a temperature ~f 570 C7
that gas contains 10.1 vol.% HF. The temperature ln the
circulating fluidized bed is 530C. The mean density of
the suspension is about 150 kg/m reactor volume~ The
quanity of solids circulated per hour through the fluidized
bed reactor:1, the recycling cyclone 2 and the re~ycling
line 3 is about 50 times~the quantity of solids contained
in the fluidized bed reactor 1.
~ he exhaust gas which has left the circula~ing
fluidized bed system is at a temperature of 530 C a~d
contains 7.5~vol.% HF as it enters~khe venturi suspenslon
heat exchanger~5.~In that heat:exchanger, solids at tO0 C, ; ;
;:wh;Lch have been:supplled~from ~he fluidLze~ bed reactor 6
th:rough l~ne 7~at~a:rate of 1797 kg/h~, are added to~the :
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exhaust gas, which ~ thus cooled to 453 C. The solids
collected in the cyclone separator 9 are supplled through
line 10 to the fluidized bed reactor l of the circulating
fluidized bed system.
The exhaust gas from the cyc:lone separator 9
or alumina hydrate
contains 4.0 vol.% HF. Fresh alumlnum hydroxlde/having a
moisture content of 12.0~ is supplied through line 22 at a
rate of 1030 kgJh. Solids at 100C are entrained at a
rate of 16,500 kg~h in lin`e 12 by the fluidizing air from
the fluidized bed cooler 6. In the venturi suspension heat
exGhanger 11, said fxesh aluminum hydroxlde and said solid
are added to the exhaust gas from the cyclone separator 9
so tnat said exhaust gas i9 cooled to 220 C. After
~eing passed through the ayclone separator 14 and the ine
exhaust gas
dust collector 16 the/is supplied to a dry scrubber.
The exhaust gas becomes avallable at a rate of 8100 mN3th
and contains 0.15 vol. ~ HF.
The solids which have been collected in 14 and 16
become available at a temperature of 220 C and are cooled
to 100C in the fluidi~ed bed cooler 6 and are su~sequently
distributed between line 7 and 12 as descrlbed hereinbefore,
in a ratio of 1:9.2.
,
Aluminum fluoride 2roduct at a rate of 937 kg/h ~
is withdrawn from~:the fluidlzed bed reactor 1 through line ::
23. That product has:a pu~lty of:91% (balance Al203 and
igniti:on:loss).
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In thls Example, an additional cooling with air is
effected in the process in accordance with the invention.
The conditions regarding the supply of gas and the operation
of the circulating fluidizad bed system are the same as
in Examole 1.
As in Example 1, the exhaust gas $rom the circulating
fluLdized bed system is at a temperature o~ ~30C as it
enters the venturi suspension heat exchanger 5, in which
solids at 100C, which have been supplied at a rat~ of
1786 kg/h from tne fluidized ~ed cooler 6 through line 7,
are added to the exhaust gaq as well as air at 40C~ whioh
is supplied at a rate of 1052 ~ 3/h through line 24. The
gas is thu3 cooled to 420 C. Sollds colleotad in the
cyclone separator 9 are supplied throu~h line 10 to the
fluidized :~ed reactor 1 of the circulating fluidized ~ed
syqt2m.
The axhaust gas from the cyclone ssparator 9
,or alumina hydrate
contalns 2.0 vol.% ~F. Fresh aluminum hydroxide'having a
moisture content of~12.0~ is su~plled through line 22 at a
rate of 1D30 kg/ll. Solids at 100 C are entrained at a rate
of 15,900 kg/h in line 12 by the~fluidizing air from the
fluid1zed bed cooler 6. In the venturi suspension~heat
excnanger 11,~said fresh aluminum hydroxide and said ~olids
are~added to~the ex~aust gas from the~cycione~separator~9 ~
o
~ so that sai~d~e~laust gas is;~cooled~to~220 C.~After~ belng ~
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pa~sed through the cyclone separator 14 and the finedust collector 16 the exhaust gas is supplied to
a dry serubber. The exhaust gas becomes available
3 and
at a rate of 5150 mN /h;/contains 0.13 vol. % HF.
The solids which have been collected in 14
and 16 become arailable at a temperature of 220 C and
are cooled to 100 C in the fluidized bed cooler 6 and
are subsequently distributed between l1nes 7 and 12 as
described hereinbefore, at a ratio of 1:8.9.
Aluminum fluoride product at a rate of 937 kg/h
is withdrawn from the fluidized bed reactor 1 through
line 23. That product has a purity of 91 % (balance Al203
and ignition loss),
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